gpio: mxc: Unlock on error path in mxc_flip_edge()

We recently added locking to this function but one error path was
over looked.  Drop the lock before returning.

Fixes: e5464277 ("gpio: mxc: Protect GPIO irqchip RMW with bgpio spinlock")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>